1. Field of the Invention
The present invention relates to a method of removing sticky contaminants, such as hot melt and pressure-sensitive adhesives and lattices (referred to as "stickies"), from waste paper (secondary fiber). More particularly, this invention relates to removing stickies from secondary fibers contaminated therewith by introducing a magnetic promoter and subjecting a slurry of the fibers to a magnetic field.
2. Description of the Prior Art
Waste paper, also known as secondary fiber, has long served as a source of raw fiber material in papermaking. Waste paper materials invariably contain one or more contaminants including inks, dye colors, fluorescent whitening agents, and "stickies" (sticky or tacky contaminants including adhesives, binders, plastic films, coatings, and the like). Sorted waste paper has had most of these contaminated papers removed and represents a higher, more expensive grade of waste paper. The growing utilization of secondary fibers in many types of paper products, however, has made it necessary for paper manufacturers to process lower grades of waste paper (i.e., unsorted waste paper), which includes a significant volume of contaminated papers. While various methods have been employed to remove the contaminants to permit incorporation of the secondary fibers with virgin pulp in papermaking, such lower grade furnish is more heterogeneous and typically contains significantly more contaminants than a higher quality waste paper. Conventional treatment methods may not be adequate to permit incorporation of a significant percentage of unsorted waste papers.
Current approaches to processing recycled fibers can be classified as repulping (sluicing of fibers and partial detachment of contaminants from fibers), coarse and fine screening (separation of fibers from contaminants by size and shape), centrifugal cleaning (separation based on density differences relative to fibers and reduction in size of contaminants by mechanical action), flotation (separation by preferential adsorption of detached contaminants onto air bubbles), washing (separation of small entrained particles from fibers by relative flow of water passing by the fibers) and refining. There is an optimum particle size range for separation of particles from fibers in each of these processes. Depending upon the specific cleanliness requirements for the decontaminated pulp, it may take a combination of most or all of these processes to cover the size range of particles that one typically encounters. Both the washing and flotation processes depend on the proper use of surfactants. Depending on the relative strength and size of the hydrophilic versus hydrophobic portion of the surfactant molecule, the surfactant will cluster around ink and other contaminant particles, rendering the particles either hydrophilic (for washing) or more hydrophobic (for flotation). The opposing natures of washing surfactants and flotation surfactants can cause problems in combination flotation/washing systems.
Certain specific removal approaches have been disclosed for specific waste paper contaminants:
U.S. Pat. No. 5,211,809 discloses removing color from dyes from secondary pulps with non-chlorine based bleaching agents in treating sequences using oxygen with combinations of peroxide, ozone, and/or hydrosulfite at controlled pH conditions (less than 8 or greater than 10).
Also, published Japanese Patent Application No. HEI 3[1991]-199477 teaches a method of recycling used paper containing either fluorescent white paper or colored paper or both by introducing ozone into a dispersed slurry of said used paper.
The most common removal problem is with inks. Printing inks have been classified broadly as impact and nonimpact inks. Impact inks are used in conventional printing processes such as letterpress, flexography, and lithography. These inks are pressed or laid onto the paper but do not fuse with it. They generally consist of a pigment suspended in an oil-based alkaline aqueous medium. The paper industry has been successfully deinking papers containing impact inks for years using washing and/or flotation type systems. Also, U.S. Pat. No. 4,381,969 teaches bleaching waste paper containing encapsulated constituents such as inks by repulping the waste paper in the presence of an aqueous alkaline solution which contains a peroxide compound such as hydrogen peroxide.
Nonimpact inks, comprised of a pigment and a thermoplastic resin, are used in reprographic printing processes. The resin is a bonding agent which fuses the pigment to the sheet and to other pigment particles. The pigments employed in nonimpact inks can be categorized as either iron-based or non-iron based (e.g., carbon based). The resin polymers become cross-linked and resistant to chemical and mechanical action, making nonimpact printed papers difficult to deink by conventional deinking processes. Once detached from the fibers the toner ink particles tend toward a size which is larger than that which can be efficiently handled by flotation or washing and too small to be removed by cleaners and screens. A list of approaches disclose methods for removing both impact and non-impact (reprographic) type inks are set forth in the co-pending parent applications noted above, and such disclosure is incorporated herein by reference.
While waste paper contaminants such as inks, dyes, and whiteners present real problems in recycling, stickies present unique problems due to their non-magnetic character and to their different chemistry and physical properties (e.g., high tackiness) compared to both impact and non-impact inks, whether magnetic or non-magnetic. In paper recycling, sticky contaminants, such as hot melt adhesives, latex, pressure-sensitive adhesives, and wax, pose considerable problems to papermakers. When present in unacceptable amounts, stickies can cause problems with both paper machine operation and product quality. This problem has been addressed by the prior art.
For example, U.S. Pat. No. 5,213,661 teaches using oxygen to reduce the tackiness of stickies in secondary pulps and, optionally, using oxygen with alkali and/or detackification agents for optimum stickies control. U.S. Pat. No. 5,080,759 teaches introducing a water-soluble organotitanium compound into the water system of a papermaking process containing the secondary fiber to reduce the tackiness and adhesive properties of the stickies contaminants.
Interestingly, U.S. Pat. No. 4,176,054 teaches separating a binder adhesive from the remaining paper material in waste paper by providing a magnetic material in the binding adhesive composition at the time of the binding operation in preparing books, catalogues, directories, pamphlets, magazines and the like so that, upon subsequent recycling, the cut up pieces of the books and magazines can be subjected to a magnetic field. Thus, the patented process is also a manufacturing operation in addition to a recycling method, and the taught recycling method is dependent upon initial incorporation of magnetic material in the hot melt or other type of adhesive used in the manufacturing process.
In actual practice, stickles are controlled by: furnish selection (purchase of hand-sorted waste paper); improved pulping and deflaking; well-designed screening/cleaning systems and dispersion; and use of additives to detackify and/or stabilize stickies. These approaches, alone or in combination, either require additional modifications in the recycling equipment or use undesirable chemical additives to control the stickies. Furthermore, these methods are not foolproof in removing the sticky contaminants.
Therefore, the object of this invention is to provide a method of removing stickies from repulped waste papers of improved efficiency and economy which is applicable to all types of stickies contaminants.